The invention relates to a picture display device comprising:
a cathode ray tube having an elongated display screen with a long axis and a short axis, a cone portion whose cross-section has an elongated shape with a long axis and a short axis, a neck comprising means for generating at least one electron beam, and
a deflection system mounted on said cone portion for generating electromagnetic fields for deflecting said electron beam(s).
A picture display device as described above is known from U.S. Pat. No. 5,962,964. The CRT of said known display device comprises a cone portion whose cross-section varies gradually from a circular shape at the neck end of the cone portion to a rectangular shape at the display screen end of the cone portion.
At the reference deflection planexe2x80x94which is the plane perpendicular to the cathode ray tube axis and going through the point of intersection between the cathode ray tube axis and the asymptote to the trajectory of the electron beam when deflected to a corner of the display screenxe2x80x94the cone portion of said known display device has a cross-section which has a substantially rectangular shape. The deflection system can therefore be positioned closer to the envelope of the electron beam(s) than within CRTs whose cones have circular cross-sections. Magnetic losses are thereby reduced and, as a result, less deflection power is needed.
According to U.S. Pat. No. 5,962,964, deflection power consumption reductions between 17% and 25% can be achieved.
Nevertheless, there is a wish to further reduce the power consumption of the deflection system.
It is an object of the invention to provide a picture display device with which a further reduction of the deflection power is achieved.
In accordance with an aspect of the invention, the picture display device is therefore characterized in that the deflection system is arranged to scan the electron beam(s) along lines substantially parallel to the short axis of the display screen, and in that the part of the cone portion which is under the deflection system has at least one cross-section whose internal outline has a long axis/short axis ratio (Ac) which is larger than or equal to the long axis/short axis ratio (Ascr) of the display screen.
The present invention allows a further reduction of deflection power of about 30% as compared with the cited prior art. This reduction is essentially achieved by reducing the line deflection power, which is the major consumer in the deflection system.
The line deflection power is reduced by reducing the sweep amplitude, which is achieved by scanning the lines substantially parallel to the short axis of the display screen (called transposed scan) instead of parallel to the long axis of the display screen (called normal scan) and the line deflection power is further reduced by reducing magnetic losses, which is achieved by bringing the line deflection system closer to the electron beam envelope, while paying particular attention to the aspect ratio of the latter.
The inventors have realised that, with transposed scanning, the aspect ratio of the cross-section of the electron beam envelope (Ael) in parts of the region under the deflection system is larger than or equal to Ascr, in contrast to normal scanning.
Based on this insight, the picture display device in accordance with the invention comprises a cone portion whose cross-section in parts of the region under the deflection system has an aspect ratio (Ac) which is also larger than or equal to Ascr.
In preferred embodiments, Ael exceeds Ascr in parts of the region under the deflection system to the extent that (Aelxe2x88x921)/(Ascrxe2x88x921)xe2x89xa71.1, and therefore it is advantageous that Ac also exceeds Ascr to the extent that (Acxe2x88x921)/(Ascrxe2x88x921)xe2x89xa71.1 in parts of that region.
In the region between the reference deflection plane and that end of the deflection system nearest to the display screenxe2x80x94which is the region where most of the magnetic field is concentratedxe2x80x94it is also advantageous that the cross-section of the cone has a shape which follows the shape of the electron beam envelope as closely as possible. Therefore, it is advantageous that, in this region, Ac first increases, goes through a maximum and then decreases.
Furthermore, the reduction of deflection power grows with growing screen aspect ratios. The invention is thus particularly effective for picture display devices with large screen aspect ratios. In particular, the invention is advantageous for picture display devices with Ascr greater than 4/3, and a fortiori for picture display devices with Ascrxe2x89xa716/9.
The economy of deflection power may be used advantageously to increase the maximum deflection angle of the electron beam(s). In preferred embodiments, maximum deflection angles larger than or equal to 120xc2x0 are realised. This is useful in building slimmer CRTs.